Image yielding layers



United States Patent 3,284,206 IMAGE YIELDING LAYERS Ralph KingsleyBlake, Westfield, N.J., assignor to E. I. du Pont de Nemours andCompany, Wilmington, Del., a corporation of Delaware No Drawing. FiledOct. 13, 1964, Ser. No. 403,661 10 Claims. (Cl. 96--107) Thisapplication is a continuation-in-part of Blake Ser. No. 236,420,applicants prior application filed Nov. 8, 1962, and issued Nov. 3,1964, as US. Patent 3,155,507.

This invention relates to photography and more particularly to newimage-yielding photographic silver halid emulsion layers and tophotographic elements embodying such la ers.

The principal processes of photography are based on the use ofcolloid-silver halide emulsion layers. In the prior art processes alatent image is formed by image-wise exposure of a radiation-sensitivesilver halide emulsion layer. Silver halide bearing a latent image hasbeen developed to silver by selective reduction in these instances. Inthe prior processes of photography the unreduced silver remaining afterdevelopment has been removed by silver halide solvents or renderedinsensitive or transparent by treatment with complexing agents. Optionalafter-treatments include intensification, reduction, toning and tinting.However, the primary or first step in image formation always has beenbased on the selective reduction step.

An entirely different type of photographic process has been described inassignees copending application of Blake, U.S. Ser. No. 236,420, filedNov. 8, 1962. The novel process of said application, characterized asphotosolubilization, requires the use of a specially prepared silverhalide emulsion layer containing a stipulated amount of an organiccompound which modifies the silver halide solubility so that, inconventional silver halide solvents, said organic compound causes thesilver halide grains to dissolve more slowly than normal. Such anelement is given an image-wise exposure and the exposed areas can thenbe treated in a solution of a silver halide solvent to yield a positive,silver halide image (the silver halide remaining undissolved in theunexposed areas). As an optional additional processing step, the silverhalide image may be intensified, e.g., by reduction, to convert it intoa black, metallic silver image.

It is an object of this invention to provide new photographic silverhalide compositions, photographic layers, and photographic elementsbearing a layer of silver halide. Another object is to provide processesfor making these products. A further object is to provide such productswhich are adapted to more versatile processes for forming silver andother images and which are simple, dependable and give results equal inquality to the prior conventional methods. A more particular object isto provide new compositions and elements for photosolubilizationprocessing embodying a class of organic compounds which provide a newcombination for the required alteration of the silver halide crystals.Still further objects will be apparent from the following description ofthe invention.

The above objects are realized in a photographic emulsion layercomprising, before exposure to actinic radiation, light-sensitive silverhalide crystals having associated therewith a silver selenide of aselenyl compound (selenol) of the formula:

where R is an organic radical, said silver selenide being of lowersolubility in water than silver chloride and the silver halide crystalsso associated with the silver selenide dissolving more slowly in a 10%by weight aqueous sodium thiosulfate solution at a predetermined pH,than untreated silver halide crystals.

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It is preferred that the selenide be present in such an amount, in termsof the ratio of its weight to the surface area of said silver halidecrystals, that when admixed in such ratio with an aqueous silverchlorobrornide (70/ 30 mole percent) gelatin dispersion containing 10 g.of gelatin per mole of Ag and .57 mg. of Ag per ml., and said silverchlorobromide dispersion is treated with 10%, by weight, aqueous sodiumthiosulfate (so that the resulting mixture contains 0.29 mg. of silverand 100 mg. of sodium thiosulfate), at least three times the amount ofsilver chlorobromide remains undissolved as in a similar dispersionsuccessively treated with 5%, by Weight, aqueous sodium hypochlorite andby weight, aqueous sodium thiosulfate (so that the resulting mixturecontains 0.29 mg.

of silver, 25 mg. of sodium hypochlorite and mg. of sodium thiosulfate),after vigorous agitation of both dispersions for 30 seconds at 25 C.

In an important embodiment of the invention, the organic selenylcompound has the formula:

where R and R, which may be the same or different, are hydrogen,hydrocarbon or substituted hydrocarbon nuclei of 1-14 carbons, i.e.,alkyl, aryl, alkaryl, aralkyl radicals, and such radicals containingsubstituent groups such as nitro, halogen, e.g., chlorine and bromine,alkoxy of 1-6 carbon atoms. When separate, at least one radical R or Ris hydrocarbon of 3-14 carbons. When R and R are taken together, theyform an aromatic or alicyclic ring. The silver salt of the selenol is ofless solubility in water than silver chloride and the silver halidecrystals so associated with the said silver salt dissolve more slowly in10% aqueous sodium thiosulfate solution, at a predetermined pH, thanuntreated silver halide crystals.

R of Formula 2 preferably is H, and R is an unsubstit-uted hydrocarbonradical of 6-10 carbon atoms and has a cyclic hydrocarbon radical of 6carbon atoms attached through a cyclic carbon of said radical to the4carbon atom of the thiazole ring. Suitable radicals of the latter typeinclude cyclohexyl, phenyl, and alphanaphthyl.

Preferably, the silver halide crystals are dispersed in awater-permeable organic colloid to form a. light sensitive photographicemulsion. The selected selenyl compound can be added to the silverhalide emulsion while the latter is inthe liquid state or the emulsionmay be coated on a suitable support and the resulting element bathed orimpregnated with a solution, e..g., an alcoholic solution of the organiccompound. The desired amount of the selenyl compound in the silverhalide emulsion may vary with a number of factors such as the nature ofthe selenyl compound and the size of the silver halide crystal (and thusthe surface area of the crystal per mole of silver halide).

When the selenol is added to the emulsion in the liquid state it is mostefiiciently adsorbed to the silver halide crystals by digesting theemulsion, i.e., heating the emulsion between and F. The selenol is usedin greater than fog-inhibiting amounts, e.g., from 0.3 to 1.5 grams permole of silver halide and, more preferably, from. 0.4 to 1.2 grams permole of silver halide. The optimum concentration of selenol is decreasedsomewhat, e.g., about 10%, when the emulsion is sensitized with aphotographic optical sensitizing dye as disclosed in assignees copendingapplication of Blake, Ser. No. 390,- 460, filed August 18, 1964.

The gelatin-silver halide ratio is quite flexible and may vary from 3 :1to 1:20 depending on the particular organic compound and application.

sulfate, at some pH between 1 and 13.

In an important use of the products of the invention, dire-ct positiveimages are formed by a process which comprises:

(a) Exposing imagewise to actinic radiation a photosensitive layercomprising silver halide crystals treated with the selenol as describedabove.

(b) Treating the exposed layer in a solution of a silver halide solventto remove soluble silver halide in the exposed image areas, thus forminga positive silver halide image, and optionally,

(c) Washing the resulting layers. If desired, the silver halide imagemay be viewed directly, eg, by projection (if on a transparent support)or it may be intensified, e.g., y

((1) Converting the residual silver halide to silver by treatment in afogging developer, e.g., a high pH, l-phenyl-4-methyl-3-pyrazolidone/hydroquinone developer containing iodide ion orby fogging the emulsion by exposure to light and then treating with asilver halide reducing agent, e.g., a conventional silver halidedeveloper, and

(e) Washing the developed layer to reveal a positive silver image in theoriginal non-exposed areas.

The imagewise solution of the exposed silver halide/ selenol stratum maybe effected by the silver halide solvents commonly used as photographicfixing agents, e.g., sodium thiosulfate, sodium thiocyanate concentratedsolutions of potassium bromide, etc. In assignees copending applicationHunt, Ser. No. 388,919 filed August 11, 1964, it is disclosed that moreefficient removal of exposed silver halide crystals (insolubilized bymercapto compounds) may be obtained when the fixing solution contains aninorganic cation selected from the class consisting of potassium,cesium, rubidium, thallium (I) strontium, and lead (II). These cationsare also effective as additives for fixing solutions used in treatingthe elements of this invention.

Reduction of the treated residual silver halide may be accomplished bythe use of any chemical reducing agent capable of reducing silver ion tosilver metal, e.g., hydroquinone, metol, sodium hydrosulfite andstannous chloride. The function of the reducing agent may be enhanced bymodifying the surface properties of the treated, residual silver halidecrystals by means of alcohol, thiourea, potase.--g., with sodiumsulfide, sodium selenide, etc. tion, color images may be obtained bydeveloping the .treated, residual silver halide with a primary aromaticamine color developing agent in the presence of a color couplingcompound either in the developing bath or previously incorporated in theemulsion. Additionally, the

silver halide image may be intensified by dye mordanting.

The present invention is not limited to a narrow class of selenols withwhich the silver halide crystals are intiniately associated or may betreated in preparing the novel compositions of this invention. Instead,a large number of useful compounds can be employed and their utility canbe readily determined by a relatively simple test. Essentially, the testconsists of two steps. Test A and Test B. In Test A, the candidateselenol must render a dispersion of silver halide crystals insoluble ina silver halide solvent, i.e., an aqueous solution of sodium thio- Ifthe candidate compound meets the insolubility requirements of Test A, itmust also meet the requirements of Test B by forming with saiddispersion of silver halide crystals a reaction product which, upontreatment with an aqueous solution of sodium hypochlorite, becomessoluble when subsequently treated wit-h aqueous sodium thiosulfate. Thefollowing practical tests are provided in further exemplification of theinvention and include specific concentrations of solutions, times, etc.,so that suitable selenols .may be readily and positively identified.

TEST A A solution, nearly saturated at 25 C. with a candidate selenol,is prepared using ethanol, acetone, dimethyl .sium iodide, etc. Thesilver halide image may be toned, H In addiformamide, water or othersuitable solvent. Depending on the solubility, a solution concentrationfrom 0.01 to 10 percent by weight is obtained. Twenty-five ml. of asilver chlorobromide dispersion containing 25 mg. of silver halide(calculated as silver bromide), prepared as described below, is treatedwith small increments (i.e., about 0.1 to 0.2 ml. at a time) of the saidcandidate solution under safelight conditions (Wratten 1A filter orequivalent) until the silver halide dispersion either is renderedinsoluble in 10% aqueous sodium thiosulfate or the candidate is foundnot to cause insolubilization. Generally insolubiliZatio-n will occurupon the addition of 0.05 g. or less of said candidate selenol,calculated as the pure compound. Compounds which must be used insubstantially greater quantities than this, e.g., 1-2 g., to effectinsolubilization are considered less preferred compounds. The silverhalide dispersion insolubility is determined by taking a 0.5 ml. portionof the silver halide dispersion (after each incremental addition of thecandidate selenol), adding about 0.1 to 0.2 ml. of 10% aqueous sodiumthiosulfate solution and observing the turbidity after 30 seconds.

As a control, one should use 25 ml. of water to which small incrementsof the candidate solution are added. Half-milliliter portions of thecontrol are treated in the same manner with the sodium thiosulfatesolution. The presence of visual turbidity relative to the control issufficient to satisfy the definition of insolubility in this test.

This test may be repeated for various pH increments from 1 to 13.Although there is some optimum pH value at which the test is mostsensitive, this is not a sharp maximum which must be precisely attained.Rather, it has been found that there is a fairly broad range of pHvalues (e.g., 2.0 to 3.0 pH units) over which the test has asatisfactory sensitivity. In practice, the silver halide dispersionmight be tested without adjustment (e.g., at pH 5.0 to 7.0) and ifinsolubilization occurs here, Test A is completed. If there is noinsolubilization, the test is repeated at a higher pH (e.g., from pH10-13). If there is still no insolublization, the test is conducted withthe emulsion adjusted to a lower pH (e.'g., about pH 1-3). Thus threedifferent pH values represent a practical maximum number which must beinvestigated to determine whether or not insolubilization will occur.

TEST B A selenol capable of insolubilizing a silver halide dispersionaccording to Test A is now ready for the next test, which again will beconducted under safelight conditions. To the above silver halidedispersion, there is added the minimum amount of a solution of thecandidate selenol found necessary for insolubilization. Halfmillilitersamples of the dispersion containing 0.5 mg. of AgBr or 0.29 mg. of Agare placed in two test tubes. To one sample is added 0.5 ml. of water;to the other is added 0.5 ml. of a 5% by weight aqueous solution ofsodium hypochlorite (25 mg. NaOCl). Next, there is added to bothsamples, 1.0 ml. of an aqueous 10% by weight solution of sodiumthiosulfate mg. thiosulfate). If, after standing for up to thirtyseconds, the sample treated with sodium hypochloriate clarifies (orbecomes less turbid) relative to the control sample, the candidateselenol meets the requirements of Test B and is satisfactory for use inaccordance with this invention. The chemical testing for selectingsuitable compounds has been found to give absolute correlation, i.e.,organic compounds which have been subjected to Tests A and B haveby-product salt by a coagulation and wash procedure as taught in Walleret al., US. Patent 2,489,341, wherein the silver halide and most of thegelatin were coagulated by an anionic wetting agent, sodium laurylsulfate, using an acid coagulation environment. Following the washingstep, the emulsion coagulant was redispersed in water. Assuming a lossof grams of gelatin per mole of silver halide during washing, the finalgelatin concentration was about 10 grams per mole of silver halide. Foruse in the \above test, the dispersion was diluted to the extent thatone milliliter of dispersion contained one mg. of silver halide(calculated as AgBr, or 0.58 mg. of Ag).

Dispersed crystals of silver halide, treated with an appropriate amountof a suitable selenol, are affected by exposure of a portion of saidcrystals to iactinic radiation, e. g., ultraviolet, visible, infrared,X-radiation, etc., to such an extent that at least of the less soluble(unexposed) crystals remain when 90% of the more soluble (exposed)crystals dissolve when treated in 10% by weight aqueous sodiumthiosulfate solution.

Suitable elements in this invention can be prepared by bathing aphotographic film in a solution of an appropriate selenol. In thisembodiment, the silver halide crystals near the surface of the coatedemulsion stratum are in contact with a higher concentration of theselenol. Crystals farther from the surface are treated with less of theselenol and, if the rate of diffusion is sufliciently slow, there may beconsiderably less of the selenol (even appreaching zero) reacting withthe lower than with the surface silver halide crystals. In suchelements, satisfactory results might be obtained with only a fraction,e.g., one-half of the amount of the selenol theoretically calculated asrequired to just cover the surface of a mole of the silver halidecrystals.

The invention will be further illustrated by, but is not intended to belimited to, the following examples.

Example I ZSELENYLBENZOTHIAZOLE To a solution of sodium selenide g, 0.2moles) in 250 ml. of water, under a nitrogen atmosphere, are added 33.9g. (0.2 mole) of Z-chlorobenzothiazole and with rapid stirring themixture is then heated to reflux. The mixture is maintained at gentleebullition for a period of to minutes until droplets of theZ-chlorobenzothiazole disappear in the reflux. The2-selenylbenzothiazole (yellow crystals of melting point 140143 C.) isthen precipitated from the solution of its sodium salt with anacidifying agent, sulfuric acid.

A lithographic emulsion containing about 10 g. of gelatin per mole ofsilver halide (prepared as described following Test B) plus 47 g. ofgelatin, was adjusted to 2320 g. by addition of water and thetemperature was adjusted to 110 F. The 2-selenylbenzothiazole, preparedas described above, was added to the emulsion from a 1% by weightethanol solution in the amount of 1.26 g. per mole of silver halide. Theemulsion was digested for 20 min. at 160 F. and then cooled to 95 F.Chrome alum hardener and other emulsion adjuvants were added as finaladditions. This emulsion was applied at a coating weight of 70 mg. ofsilver halide (calculated as AgBr) per square decimeter on 0.004-inchthick polyester photographic film base as described in Example IV ofAlles US. 2,779,684. The coating, after imagewise exposure, showed )8.greater rate of dissolution in a 1.0 N (0.5 molar) aqueous sodiumthiosulfate solution in the exposed areas than in the unexposed areas soas to form a :positive silver halide image. Subsequent flashing to whitelight, followed by treatment with a reducing agent (a conventionalphotographic developing solution containing1-phenyl-4-methyl-3-pyraz0lidone and hydroquinone), resulted in theformation of an intensified positive image of metallic silver.

Example II .SELENOPHENOL Commercially available selenophenol was testedaccording to the above-described Test A and it produced the requiredinsolubilization of silver halide crystals. Chemical solubilization alsooccurred according to Test B as described above. More quantitativeresults wene obtained by applying Test C, a simulated photographic testas described below:

TEST C A 0.5 ml. portion of the insolubilized dispersion prepared inTest A under safelight conditions is placed in a 12x75 mm.heat-resistant glass tube 3 inches from a highintensity tungstenfilament incandescent lamp (General Electric Reflector Photoflood Lamp,No. PH/RFLZ). This insolubilized dispersion is exposed to the lamp forup to 10 minutes. A control consisting of another 0.5 ml. portion of theinsolubilized silver halide dispersion from Test A is taken undersafelight conditions. Two-tenths of a milliliter of 10% aqueous sodiumthiosulfate is added to each of the dispersion samples which arecompared under safelight conditions. Any reduction in turbidity of thedispersion exposed to the photoflood lamp compared to the unexposedcontrol after treatment with aqueous sodium thiosulfate solution showsthat photosolubilization occurs.

In this more quantitative Test C, it was determined that the approximateminimum quantity of selenophenol required to i-nsolubilize 25 milligramsof silver halide was 0.4 milligram.

Example III 2-snLENYLBENZOSELENAZOLE According to a procedure like thatdescribed in the Example I, 2-selenylbenzoselenazole was prepared using2-chlorobenzoselenazole in place of the 2-chlorobenzoth'iazole of thatexample.

A photographic element was prepared by coating an aqueous gelatindispersion of silver chlorobromide (70 mole perwnt silver chloride and30 mole percent silver bromide) on a film base prepared as described inExample IV of Alles US. Patent 2,779,684. The dispersion had a ratio ofsilver halide to gelatin of 19:1 by Weight and was coated at a pH of 6at a rate of 116 milligrams of silver halide per square deci-meter.After drying, the element was bathed for about 30 seconds in anethanolwater solution of 2-selenylbenzoselenazole and dried. Thesolution of Z-selenylbenzoselenazole was prepared by diluting 5 ml. of astock solution (1 gram of the compound made up to ml. in ethanol) withan additional 20 ml. of ethanol and 10 ml. of water. The dried elementwas then exposed behind a photographic transparency for 5 seconds to theradiation from the lamp described in Example lI spaced at a distance ofabout 6 inches. The exposed element was then immersed in a solutioncontaining, on a liter basis, 77 g. Na S O 7.5 g. Na SO 9 g. Na B O''10H O, 6 ml. glacial acetic acid, 10 g. KAl (SO -12H O and 10 g. CHCOOK. A 30-second treatmentin this solution resulted in the removal ofthe silver salt in the exposed areas. Subsequently, the fixed film wasthen rinsed briefly in water and bathed in a rapid acting foggingphotographic developer solution comprising1-phenyl-4-methyl-3-pyrazolidone and hydroquinone as reducing agents towhich there had been added potassium iodide and a direct positive imageformed. All of the above operations were carried out under red safelightillumination.

Example I V Z-SELENYLBENZOXAZOLE The above compound was preparedaccording to the procedure of Example I except for using2-chlorobenzoxazole in place of the 2-chlorobenzothiazole of thatexample.

-chlorobenzoxazole is commercially available. The

aaeaeos selenyl compound thus prepared was tested as a treating solutionfor a photographic element as described in Example III and a similarpositive image was obtained.

This selenol also caused the required insolubilization of silver halidecrystals according to Test A and the required chemical solubilizationaccording to Test B. In Test C it was determined that the approximateminimum quantity of 2-selenylbenzoxazole required to insolubilize the 25mg. of silver halide was 1.0 mg.

Example V 2-SELENYL-4t-PHENYLTHIAZOLE 2-bromo-4-phenylthiazole wasprepared according to the procedure of Ralhan et al., J. Indian Chem.Soc., vol. 37, No. 12 (1960) page 773-4. One-tenth of a mole of thiscompound was added to a slight excess of an ether solution ofn-butyllithium at -l to C. (using a Dry Ice-acetone bath). After aboutan hour and a half of stirring at this temperature, the reaction mixturewas cooled to C. and 0.1 mole of powdered selenium was added. Thereaction mixture was then stirred while allowing it to warm to about 0C., then the mixture was poured onto ice. Upon acidification with H 80the desired compound, 2-selenyl-4-phenylthiazole was isolated from theseparated ether layer.

The resulting compound, was tested as a treating solution for aphotographic element as described in Example III and a similar positiveimage was obtained.

This selenol also caused the required insolubilization of silver halidecrystals of Test A and the required chemical solubilization of Test B.In Test C it was determined that the approximate minimum quantity of2-selenyl-4- phenylthiazole required to insolubilize the 25 mg. ofsilver halide was 0.3 mg.

Example VI Z-SELENYL-l-CY CLOHEXYLTHIAZOLE The above compound wasprepared using a procedure very similar to that of Example V except forthe substitution of Z-bromo-4-cyclohexylthiazole in place of 2-bromo-4-phenylthiazole.

The resulting compound was tested as a treating solution for aphotographic element as described in Example III and a similar positiveimage was obtained.

Example. VII

2-S-ELENYL-ei-AMYLTHIAZOLE The above compound was prepared using aprocedure very similar to that of Example V except for the substitutionof 2-bromo-4-amylthiazole in place of 2-bromo-4- phenylthiazole.

The resulting compound was tested as a treating solution for aphotographic element as described in Example III and a similar positiveimage was obtained.

Example VIII LSELENYLOCTANE l-selenyloctane was prepared from normalbromooctane by reaction with sodium selenide under nitrogen. Theresulting compound was tested as a treating solution for a photographicelement as described in Example III and a similar positive image wasobtained.

A number of other selenols of the structure R-SeH can be synthesizedaccording to procedures similar to those described above and can be usedin the preparation of photosolubilizable compositions and elements togive photographic results as described in the above examples. Suchcompounds include:

2-selenyl-4-(p-bromophenyl) thiazole 2-selenyl-4-(p-methoxyphenyl)thiazole 2-selenyl-4-(p-ethylphenyl) thiazole2-selenyl-4-(p-chlorophenyl) thiazole The silver halide need not be acombination of silver chloride and silver bromide, but may be silverchloride, silver bromide and other mixed halide systems conventional inphotographic practice, e.g., silver bromoiodide. While, for rapidprocessing, a high silver halide to binder ratio is desirable, moreconventional ratios can also be used.

In place of part of the gelatin, other natural or syntheticwater-permeable organic colloid binding agents can be used and in somecases such binders can be used alone. Such agents includewater-permeable or water-soluble polyvinyl alcohol and its derivatives,e.g., partially hydrolyzed polyvinyl acetates, polyvinyl ethers andacetals containing a large number of intralinear groups, hydrolyzedinterpolymers of vinyl acetate and unsaturated addition polymerizablecompounds such as maleic anhydride, acrylic and methacrylic acid estersand styrene. Suitable such colloids of the last-mentioned type weredisclosed in U.S. Patents 2,276,322; 2,276,323 and 2,397,866. The usefulpolyvinyl acetals include polyvinyl acetaldehyde acetal, polyvinylbutyraldehyde acetal and polyvinyl sodium o-sultobenzaldehyde acetal.Other useful colloid binding agents which can be used include thepoly-N-vinyllactams of Bolton U.S. Patent 2,495,918, variouspolysaccharides, e.g., dextran, dextrin, etc., the hydrophiliccopolymers in Shacklett U.S. Patent 2,833,650, hydrophilic celluloseethers and esters, and polymers of acrylic and methacrylic esters andamides. Also, it has been found practical to treat silver halide layerson a base material in the essential absence of a binder, e.g., thoseprepared by chemical or vacuum deposition.

The emulsion may optionally contain any of the usual adjuvantscustomarily employed in silver halide systems so long as they do notinterfere with the absorption and complexing action of theselenomercaptans of the invention.

The emulsions can be coated on any suitable support, e.g., celluloseesters, cellulose mixed esters; superpolymers, e.g., poly(vinyl chlorideco vinyl acetate); polyvinyl acetals, butyrals; polystyrene; polyamides,e.g., polyhexamethylene adipamide, polyesters, e.g., polycar bonates,polyethylene terephthalate/isophthalate, esters formed by condensingterephthalic acid and its derivatives, e.g., dimethyl terephthalate withpropylene glycol, diethylene glycol, tetramethylene glycol,cyclohexane-1,4- dimethanol (heXahydro-p-xylene dialcohol); paper,metal, glass, etc.

The novel photographic compositions of this invention have numerousadvantages. One advantage is the simplicity of their preparation. Thephotographic processes applicable to the compositions of the inventionlikewise have advantages over previously known systems based onselective reduction of exposed silver halide for forming either directpositive or negative image without resorting to the special effects andsensitizing procedures previously used for preparing such images. Inaddition, since direct positive image formation does not requireselective reduction, this process is not limited to the use of certainphotographic developing agents but may be accomplished by using a widerange of reducing agents. Many such compounds are of very low cost andcan be used to form images of much higher covering power than customary,thus effecting important economies in processing, as well as greatlyincreasing the efficiency of the silver image.

Another advantage of this invention is that it provides new elements forforming silver images that do not require special equipment andapparatus. A further advantags is that the elements can be usedsuccessfully by photographic technicians and photographers of ordinaryskill. A still further advantage is that the elements can be processedwith conventional reducing agents, e.g., developers and fixing agents.Still additional advantages will be apparent from the above descriptionof the invention.

What is claimed is:

1. A photographic emulsion layer comprising, before exposure to .actinicradiation, light-sensitive silver halide crystals having associatedtherewith in greater than foginhibiting amounts a silver selenide of aselenyl compound of the formula wherein R is an organic radical that isattached to the Sc atom by a carbon atom; said silver selenide being oflower solubility in Water than silver chloride, the silver halidecrystals so associated with the silver selenide dissolving more slowlyin 10% aqueous sodium thiosulfate than untreated silver halide crystalsat a predetermined pH, the selenyl compound of said formula beingpresent in such amount, in terms of the ratio of its weight to thesurface area of said silver halide crystals, that when admixed in suchratio with an aqueous silver chlorobromide (70/30 mole percent) gelatindispersion containing 10 g. of gelatin per mole of Ag and .57 mg. of Agper ml., and said silver chlorobromide dispersion is treated with 10%,by Weight, aqueous sodium thiosulfate so that the resulting mixturecontains 0.29 mg. of silver and 100 mg, of sodium thiosulfate), at leastthree times the amount of silver chlorobromide remains undissolved ascompared with a similar dispersion successively treated with 5%, byweight, aqueous sodium hypochlorite and by weight, aqueous sodiumthiosulfate (so that the resulting mixture contains 0.29 mg. of silver,25 mg. of sodium hypochlorite and 100 mg. of sodium thiosulfate), aftervigorous agitation of both dispersions for 30 seconds at 25 C.

2. An emulsion layer according to claim 1 wherein gelatin is present asa binding agent for said crystals.

3. An emulsion layer according to claim 1 wherein the silver halide issilver chlorobromide.

4. An emulsion layer according to claim 1 wherein the silver halide issilver chloride.

5. A photographic emulsion layer comprising before exposure to actinicradiation light-sensitive silver halide crystals having associatedtherewith in greater than fogin hi biting amounts a silver selenide of aselenyl compound of the formula where R and R are members selected fromthe group consisting of hydrogen and hydrocarbon nuclei of 1-14 carbonatoms connected through carbon to the thiazole ring, at least one of Rand R when separate being a hydrocarbon nucleus of 3-14 carbon atoms andwhen taken together consituting the atoms necessary to complete with thetwo carbon atoms of the thiazole ring a cyclic radical; said silverselenide being of lower solubility in water than silver chloride, thesilver halide crystals s-o associated with the silver selenidedissolving more slowly in 10% aqueous sodium thiosulfate than untreatedsilver halide crystals at a. predetermined pH, the selenyl compound ofsaid formula being present in such amount, in terms of the ratio of itsWeight to the surface area of said silver halide crystals, that whenadmixed in such ratio with an aqueous silver chlorobromide (/30 molepercent) gelatin dispersion containing 10 g. of gelatin per mole of Agand .57 mg. of Ag per ml., and said silver chlorobromide dispersion istreated with 10%, by weight, aqueous sodium thiosulfate (so that theresulting mixture contains 0.29 mg. of silver and mg. of sodiumthicsulfate), at least three times the amount of silver chlorobromideremains undissolved as compared with a similar dispersion successivelytreated with 5%, by weight, aqueous sodium hypochlorite and 10%, byweight, aqueous sodium thiosulfate (so that the resulting mixturecontains 0.29 mg. of silver, 25 mg. of sodium hypochlorite and 100 mg.of sodium thiosulfate), after vigorous agitation of both dispersions for30 seconds at 25 C.

6. An emulsion layer according to claim 5 wherein gelatin is present asa binding agent for said crystals.

7. An emulsion layer according to claim 5 wherein the silver halide issilver chlorobrornide.

8. An emulsion layer according to claim 5 wherein the silver halide issilver chloride.

9. An emulsion layer according to claim 5 wherein the selenyl compoundis 2-selenylbenzothiazole.

10. An emulsion according to claim 5 wherein the selenyl compound isZ-selenyl-4-phenylthi'azole.

References Cited by the Examiner UNITED STATES PATENTS 3/ 1940 Kendall96--10 9 12/1964 Hepher et al 9633 X References Cited by the ApplicantUNITED STATES PATENTS 2,520,358 8/1950 Brooker. 3,155,507 11/1964 Blake.

1. A PHOTOGRAPHIC EMULSION LAYER COMPRISING, BEFORE EXPOSURE TO ACTINICRADIATION, LIGHT-SENSITIVE SILVER HALIDE CRYSTALS HAVING ASSOCIATEDTHEREWITH IN GREATER THAN FOGINHIBITING AMOUNTS A SILVER SELENIDE OF ASELENYL COMPOUND OF THE FORMULA